Recombination of polynucleotides can be carried out using many methods known in the art. Traditional techniques for recombining nucleic acids have utilized restriction enzymes and ligating enzymes for the creation of novel nucleic acid molecules. Recombinant molecules such as cloning and expression vectors can be utilized to integrate a nucleic acid sequence of interest into the genome of a host cell, and/or drive the expression of one or more genes of interest. Utilization of a vector to drive expression of a gene of interest in the cell, for example a yeast cell, requires that the vector contain requisite genetic elements that enable replication and expression of the gene of interest. These elements may include, for example, the gene or genes of interest, a promoter sequence, a terminator sequence, selectable markers, integration loci, and the like.
Assembly of elements into a single vector using traditional restriction and ligation enzyme-based methods can be time-consuming and laborious. Each sub-cloning step, i.e., the introduction of a new nucleic acid fragment into an existing polynucleotide, can require that the resulting clone be screened and characterized before the introduction of additional fragments. Clones produced by blunt end ligation require confirmation that the fragment was introduced in the proper orientation. On the other hand, sticky-end ligation requires that the restriction sites utilized to produce the sticky ends on the acceptor fragment also be present in the donor fragment, but not at a site that would interrupt the sequence of interest within the donor fragment. Thus, the selection of workable restriction sites depends entirely on the compositions of the pieces being joined and must be carefully considered in each case. In addition, these methods often introduce extraneous nucleic acid sequences to the resulting clone that can interfere with the structure and function of the desired gene products. Further limiting the efficiency of restriction-enzyme based cloning methods is the intrinsic limitation on the number of nucleic acid molecules that can be ligated together in a single reaction.
The polymerase chain reaction (PCR) is a powerful technique by which specific polynucleotide sequences, including genomic DNA, cDNA and mRNA, are amplified in vitro. PCR typically comprises contacting separate complementary strands of a target nucleic acid with two oligonucleotide primers under conditions that allow for the formation of complementary primer extension products on both strands. These strands act as templates for the synthesis of copies of the desired nucleic acid sequences. By repeating the separation and synthesis steps in an automated system, exponential duplication of the target sequences can be achieved.
One method of PCR, termed “splicing by overlap extension” (“SOE”; see, e.g., U.S. Pat. No. 5,023,171), facilitates the assembly of DNA molecules at precise junctions without the use of restriction enzymes or ligase. Component fragments to be recombined are generated in separate polymerase chain reactions using uniquely designed primers which produce amplicons having complementary termini to one another. Upon mixing and denaturation of these amplicons, strands having complementary sequences at their 3′ ends overlap and act as primers for each other. Extension of this overlap by DNA polymerase produces a nucleic acid molecule in which the original sequences are “spliced” together. Subsequent rounds of PCR amplify the resulting spliced polynucleotide.
SOE, while more efficient than traditional ligation enzyme-based methods for combining a plurality of nucleic acid fragments, does require time to optimize primer sequences and amplification conditions to produce desired products. Each junction between the fragments to be spliced together must be individually considered, and a pair of primers must be designed for each fragment in order to make the ends compatible. Traditional considerations for the design of PCR primers, e.g., melting temperature, G-C content, avoidance of hairpin and dimer formation, and stringency for false priming sites, must be considered even more carefully as the number of fragments to be spliced in the SOE reaction increases.
Thus, despite advances in recombinant DNA technology, there exists a need for improved methods that provide for the rapid and ordered assembly of polynucleotides. Particularly needed are methods which can facilitate the assembly of a number of polynucleotides with minimal manipulation and characterization of intermediate products, and without the need for primer optimization steps. These and other needs can be met by compositions and methods of the present invention.